1. Field of the Invention
The present invention relates to a mass selector which selects a mass-to-charge ratio of an ion based on a relationship between a time of flight and the kinetic energy of the ion; and an ion gun, an ion irradiation apparatus and a mass microscope which use the same.
2. Description of the Related Art
A cluster ion beam can be obtained by ionizing a particle which is formed of a cluster generated from a high-pressure gas that has been injected into a vacuum from a nozzle, or a particle which is formed of a cluster that has been generated by the cooling of a vapor of a solid, through an electron impact ionization or a photo-ionization process.
In addition, the cluster ion beam formed of the clustered particles can be generated by a direct ionization of the surface of a charged droplet, a solid or liquid by field evaporation, even though the particle does not pass through the ionizing process.
A technique of irradiating the surface of a solid with the cluster ions is used in a surface processing such as etching, sputtering and film-forming. In addition, when the surface is irradiated with a cluster ion having a large mass, the cluster ion has an effect of being capable of ionizing a high polymer while suppressing the fragmentation. Accordingly, the technique is effectively applied also to a surface analysis apparatus (Japanese Patent Application Laid-Open No. 2011-29043). When the technique is applied to the surface analysis apparatus, sputtering efficiency and ionization efficiency for an object to be irradiated vary according to the size of the cluster ion. Accordingly, it is desirable that a cluster ion having a predetermined size is selected and then the object is irradiated with the selected cluster ion.
A cluster ion irradiation apparatus has a cluster ion gun, a beam control section, and an irradiation section. The cluster ion gun includes a cluster ion generation section and a mass selector. Each part is evacuated by a vacuum pump, and constitutes the whole vacuum chamber.
The cluster ions which have been generated in the cluster ion generation section usually include clusters having various sizes. Accordingly, in many cases, cluster ions having a predetermined size are selected after the generated cluster ions have been incident on the mass selector, and then the object is irradiated with the selected cluster ions.
In mass selection methods, there exist a magnetic field type (WO 10/029929), a quadrupole type, a time-of-flight type and the like, but the time-of-flight type mass selection method is suitable for the cluster ion having the large mass. The time-of-flight type mass selection method is a method which is used when the flight distance and the energy of ions are known, and is a method of selecting the ions according to the mass based on a relationship between the times of flight and the energy of the ions which have been pulsed (though reference pulse in measurement for time of flight of ion is referred to as trigger pulse, in the present invention) prior to the mass selection.
In this case, the relationship between the time of flight and the energy of the ion is expressed by Expression 1. Accordingly, a difference between masses causes a difference also between the times of flight (though this relationship is referred to as mass dispersion, in the present invention). If this relationship is used, only the ions having a particular time of flight are taken out, and thereby ions having an identical mass can be obtained.
                    Expression        ⁢                                  ⁢        1                                                                                      ⁢                              m            z                    =                      2            ⁢                                                  ⁢            e            ⁢                                                  ⁢                                          V                ⁡                                  (                                      t                    L                                    )                                            2                                                          (                  Expression          ⁢                                          ⁢          1                )            
Here, m represents the mass of an ion, z represents a valence of the ion, t represents a period of time in which the ion flies in an equipotential space, V represents a passage voltage of the ion, L represents a flight distance, and e represents an elementary electric charge.
On the other hand, in the time-of-flight type mass selection method, a relationship among the mass m and a mass difference Δm of an ion to be mass-selected and the time t of flight and the time difference Δt of the ion is expressed by Expression 2. The time difference Δt is dominated by a duration time of a trigger pulse, a measuring error in the measurement of the time of flight t, and the like.
                    Expression        ⁢                                  ⁢        2                                                                                      ⁢                                            Δ              ⁢                                                          ⁢              m                        m                    =                      2            ⁢                                          Δ                ⁢                                                                  ⁢                t                            t                                                          (                  Expression          ⁢                                          ⁢          2                )            
Here, (Δm/m) represents a mass resolution, and in order to obtain a high mass resolution, it is effective to shorten the time difference Δt by shortening the duration time of the trigger pulse.
Units which are used for generating pulsed ions include: (1) a method of using an ion source which generates ions in a pulsing way; and (2) a method of combining an ion source which generates ions continuously in terms of time (in DC-like way) with a chopper which temporarily interrupts the ions. Incidentally, in the present invention, an operation of passing ions only in fixed periods of time in a traveling direction to pulse the ions is referred to as chopping, and a mechanism of conducting the operation is referred to as a chopper.
The type of chopper is roughly classified into two types which are a retarding method (retarding) and a blanker method (blanker).
The retarding method is a method of applying a coulomb force to the ion which has been emitted from the ion source, in a direction opposite to the traveling direction of the ion, and thereby making the ion reflected.
The blanker method is a method of using a parallel flat-plate electrostatic deflector which deflects the flight direction of the ion toward a direction perpendicular to the traveling direction of the ion or a deflector which deflects the ion by a magnetic field, in combination with an aperture electrode having an opening therein through which the ion passes.
In the blanker method, when an appropriate voltage is applied to the deflector, at least a part of the ions which have been incident on the aperture electrode passes through the opening of the aperture electrode. In addition, when an appropriate voltage different from the above voltage is applied to the deflector, the ion trajectory is deflected so as to collide against the aperture electrode, and accordingly the ions do not pass through the opening. According to such an operation, the blanker method can pass the ions through the opening only in a fixed period of time in the traveling direction, or can interrupt the ions from passing through the opening.
The electrostatic type of blanker method has a feature of being capable of pulsing ions at high speed, can pulse the ions with such a short duration time as an order of microseconds or shorter, and accordingly is often used for the time-of-flight type mass selection method.
The time-of-flight mass selector can obtain a beam of cluster ions having an identical mass (can select mass) by bringing mass dispersion out in the cluster ion beam in which the cluster ions having various masses exist together therein, and taking out the cluster ions having a particular time of flight.
In the cluster ion irradiation apparatus, the cluster ions which have been mass-selected are subjected to the control of acceleration, deceleration, focusing or dispersion in the beam control section, and then, an object to be processed or a sample, which is an object to be irradiated and is arranged in the irradiation section, is irradiated with the controlled cluster ions.
As has been described above, a cluster ion beam contains clusters having various sizes before being subjected to mass selection, and accordingly the time-of-flight type mass selection method is required to have a high mass resolution.
In order to attain the high mass resolution, a mass selector needs to select cluster ions having different times of flight in such a way that the time difference is short. Accordingly, it is effective to use a blanker type of chopper for the time-of-flight type mass selection method.
On the other hand, when an object to be processed or a sample is irradiated with the cluster ion which has been mass-selected, it becomes necessary that the position on the object to be processed or the sample, which is irradiated with the cluster ion beam, is controlled with high accuracy.
This is because the accuracy of the position on the object to be processed, which is irradiated with the cluster ion beam, exerts an effect on the processing accuracy of etching or the like and the measurement accuracy of a spatial distribution of secondary ions or neutral particles when the secondary ions or the neutral particles emitted from the sample are measured.
Particularly, when the cluster ion is used for irradiating an object as a primary ion of a so-called time-of-flight secondary ion mass spectrometer (TOF-SIMS) which measures the secondary ions that have been generated by irradiation with ions, with a time-of-flight mass analyzer, it is required that the object is irradiated with the cluster ions having a short pulse in terms of time and with high positional accuracy.
However, conventionally, there has been the case where the positional accuracy is not sufficiently controlled. For instance, when the blanker type of chopper has been used, the deflector deflects the cluster ion beam to a direction perpendicular to the traveling direction of the cluster ion beam. Accordingly, a trajectory of at least a part of the cluster ion beams are changed, and the changed cluster ion beam passes through the aperture electrode diagonally with respect to the direction in which the cluster ion beam has traveled before being mass-selected. The cluster ion beam which has diagonally passed through the aperture electrode irradiates the position on an object to be irradiated, which is different from a position that an undeflected cluster ion beam has irradiated. Similarly, also in the case when the retarding type of chopper is used, the trajectory of the cluster ion beam is deflected, and thereby the accuracy of the position is lowered which is irradiated with the beam.
Accordingly, the time-of-flight mass selector having the blanker type of chopper has had such a problem that the accuracy of the position on the object to be processed or the sample decreases, which is irradiated with the cluster ion, when the mass selector selects the masses of the ions.
The present invention is designed with respect to the above described problems, and is directed at providing a time-of-flight mass selector that can irradiate an object to be processed or a sample with ions of which the masses have been selected, with high positional accuracy.